In large public facilities such as stations, airports, harbors, baseball stadiums, soccer stadiums, event halls, and museums where a large number of people gather, security is ensured by checking luggage of users upon their admittance thereto to prevent hazardous substances, e.g., explosives, to be carried therein. Conventionally, neutron has been known to enable detection of explosives. Because many explosives include nitrogen as a constituent element, it has been suggested in developments of the past to detect an explosive based on presence and concentration of nitrogen. This is achieved by allowing fast neutron from a neutron generator to pass through a moderating material, thereby moderating the fast neutron down to thermal neutron, and irradiating the explosive with thermal neutrons and detecting energy of gamma ray produced thereby. However, recently, explosives having carbon, hydrogen, or oxygen as a constituent element but not nitrogen have been developed. Such explosives produce gamma ray upon being irradiated with fast neutron from a neutron generator, because of inelastic scattering of the fast neutron and each of the constituent elements thereof. Therefore, by detecting energy of such gamma ray, an explosive can be detected based on the concentration of each of these constituent elements.
An example of such a hazardous substance detection apparatus is disclosed in Patent Document 1 listed below. A composite cavity structure used for an explosive detection method according to the Patent Document 1 can moderate neutron from a high energy neutron source down to low energy thermal neutron. Because low energy thermal neutron reacts with nuclei used heavily in a luggage or a parcel having nitrogen housed therein to produce gamma ray, the gamma ray is detected and analyzed using a detector to detect the presence and concentration of the nitrogen, thus enabling detection of the explosive.
[Patent Document 1] Japanese Patent Application Laid-open No. S64-086047